1)
What is "Derech Sha'ar Bein ha'Chomosayim", and why was it possible to flee through it?
1.
Rashi: It is a cave (tunnel) that goes from [the king's] house to Arvos Yerecho.
2.
Radak: It is a gate between two walls. [The soldiers] fled covertly through it; the Kasdim did not sense this.
3.
Malbim (Melachim II, 25:4): There were three walls around Yerushalayim. The Kasdim breached the first, and the officers camped in Sha'ar ha'Tavech. The Jewish soldiers fled from the gate between the inner walls. Malbim (here) - this fleeing is the breach mentioned in Melachim II, 25:4 and below (52:7).
